Transpiration Stream

Question 1

Transpiration Stream

Answer 1

Transpiration stream: the movement of water from the soil, through the plant, to the air surrounding the leaves – driven by water potential gradient (hydrostatic pressure gradient)

Question 2

Apoplast pathway:

Answer 2

Apoplast pathway:
Water (containing mineral ions/salt) passes through spaces in cellulose cell walls and between cells. Does not pass through any plasma membranes into cells. Water moves by mass flow down hydrostatic pressure gradient, not osmosis (most water moves via apoplast as it provides least resistance).

Question 3

Symplast pathway:

Answer 3

Water enters the cytoplasm through the partially permeable plasma membrane by osmosis, it can then pass from one cell to the next through the plasmodesmata down water potential gradient.
Plasmodesmata: gaps in the cell wall containing cytoplasm that connects two cells, cytoplasmic bridges.
- In symplast water may pass through vacuoles of cell through tonoplast – vacuolar pathway.

Question 4

Water uptake & movement across the root:

Answer 4

The epidermis (outer layer) of root hair cells actively transports mineral ions from soil into cell, lowering water potential in cell, water osmoses in.

Some water osmoses (symplast) across the root cortex cells down a water potential gradient to the endodermis of the vascular bundle.

Most water travels through the apoplast pathway to the endodermis but must then enter the symplast pathway to enter the medulla and xylem, as the apoplast pathway is blocked by the (waxy) Casparian strip.

Question 5

Role of the endodermis:

Answer 5

Drives movement of water across the root by actively pumping mineral ions (transport proteins) from cytoplasm of cortex cells to medulla cells/xylem.
- Makes the water potential of the medulla and xylem lower, so water osmoses from cortex cells into the medulla and xylem.

Casparian strip: blocks water entering the medulla through the endodermis via the apoplast pathway, water must go via the symplast pathway (blocks apoplast).
- So water & dissolved mineral ions (esp. nitrates) have to pass through the selectively permeable plasma membrane (symplast pathway) – controls what can enter (pathogens/toxic substances).
- Also prevents water passing back from the medulla into the cortex cells as the apoplast pathway is blocked, symplast not possible as water potential gradient is in other direction.

Question 6

Movement of water from roots to leaves:

Answer 6

• Water moves into xylem down water potential gradient – by osmosis.
• Hydrostatic pressure (root pressure) is high at the bottom of the xylem.
• Due to endodermis actively pumping mineral ions into xylem – lower water potential, water follows by osmosis.
• Hydrostatic pressure forces water into xylem from medulla as Casparian strip blocks it going backwards.
• Water vapour loss/transpiration in the leaves, creates low hydrostatic pressure at top of xylem, creating transpiration pull – water is pulled up in a continuous column due to cohesion/tension theory:
• The hydrostatic pressure gradient creates tension in the xylem, which pulls water up the xylem, in a continuous column (relies on cont column).
• Cohesion – attraction between water molecules (due to H bonds), which holds water in a continuous column.
• Bordered pits also ensure column is unbroken.
• Capillary action – the polar water molecules are also attracted to xylem vessel walls – adhesion. As xylem vessels are narrow, these forces of attraction pulls water up sides of vessel.
• Water moves up stem by mass flow, down hydrostatic pressure gradient.

Root (hydrostatic pressure) increases with temp/O2/respiratory substrates – suggests metabolic reactions.

Question 7

Water leaving the leaf:

Answer 7

• Water enters leaf from xylem, moves by osmosis into cells of spongy mesophyll (symplast), may also
  pass along cell walls via the apoplast pathway.
• Water evaporates from spongy mesophyll cells into sub-stomatal air space.
• Water vapour diffuses out of leaf through stomata. This relies on difference in conc of water vapour
  molecules in the leaf and outside (water vapour potential gradient).
• A small amount of water evaporates through waxy cuticle.